Frequency modulation detector



NOV. 25, 1941. CRQSBY 2,263,615

FREQUENCY MODULATION DETECTOR Filed'April 6, 1940 3 Sheets-Sheet 2 l I 4 IMO/o F/EEqwE/mr our P117 FEEQl/ENCY 0 Mam/M750 Tmventor Mwrmiy G. Crawly, BB KW (Ittorneg Nov. 25, 1941. M. G. CROSBY FREQUENCY MODULATION DETECTOR Filed April 6, 1940 3 Sheets-Sheet 3 Ep=Zpf MurruyGf C Gttorneg Patented Nov. 25,

STATES PAT ramuancr MODULATION nn'rao'roa Murray 6. Crosby, mime, N. a,

Radio of Delaware orporation of America, a corporation Application April 6, 1940, Serial No. 328,354

9 Claims.

This concerns a new and improved frequency modulation detector which uses a multi-grid tube in conjunction with a simple tuned circuit to detect the frequency variationson a frequency modulated wave. Y

The prior'art of multi-grid detectorsused in frequency modulation reception is disclosed in my United States Patent #2,087,429,- dated July 7, 1937. The invention of the present application utilizes simplified elements to perform the functions described in the patent. In the prior patout the principle of feeding the frequency modulated wave to one grid and retarded frequency mcdulatedwave to another grid is disclosed. The present application discloses a simplified method of and means for imparting the variable phase shift with frequency change to the frequency modulation supplied over the retarded branch to one grid.

In describing my invention, reference will be made to the attached drawings wherein:

Figs. 1, 3, 4, and each illustrate a modification of my new and improved frequency modulated wave demodulator. In the modification of Fig. 5, means is-provided to use the circuit to demodulate amplitude modulated waves.

Fig. 2 is a vector diagram used to illustrate the The condenser 0 taken with the tuned impedance Z provides means for shifting thephase of th voltage supplied to grid G1. Theapplied frequency modulated wave voltage is represented byE and the current through condenser C and impedance Z is represented by I. This current will lead the voltage E .by 90 degrees since C is adjusted to have a high reactance compared to the impedance Z which is tuned to the mean frequency of the applied frequency modulated wave. Since Z is tuned to the mean frequency, it will be resistive at that frequency so the phase of I will be determined by C and will be 90 degrees displaced relative to the phase of voltage E. The voltage drop Zr will be in phase with the current I as shown in Fig. 2.

operation of a phase shifter and a tuned reactance which is responsive to frequency modulations; while Figs. 6, '7, and 8 are characteristic curves illustrating the operation of the demodulator tubes used in my circuits.

Fig. 1 shows an embodiment of the invention. The frequency modulated wave may be of any mean frequency and may be fed to the leads I and .2 from high frequency amplifiers or from the intermediate-frequency amplifiers of a wave heterodyning system. The frequency modulated wave is applied across the said leads I and 2. The terminal I is connected to the grid G2 by a condenser 4 so that frequency modulated voltage is fed directly to the second grid G2 of multigrid detectortube Ill through blocking condenser 4. The same frequency modulated wave' is fed through condenser C to tuned circuit Z and the drop across tuned circuit Z is fed to the first grid G1 of detector I. The input circuits described above are completed by the connection between lead 2 and the circuit z and the cathode I2 of tube Ill. The detected output appears across resistor 20 and is fed to the utilizing device through blocking condenser 30. Bias for the required operation of tube It is provided by a resistance condenser unit RC which with unit Z and resistance R; provides direct current paths between the cathode l2 and grids G1 and G2, respectively.

In operation, the voltages fed tothe detector grids take the phase relations shown in Fig. 2.

When the frequency of the received wave deviates to either side of the mean frequency to which Z is tuned, the voltage Zr will shift to the positions Z1 or Z1", as shown in Fig. 2. This phase shift is due to the phase shift accompanying the off-tuning of the voltage wave in tuned circuit Z as the frequency of the wave is modu lated. Thus, the first grid G1 ofdetector l0 receives a voltage which varies with phase and frequency through the angles 0 or 0 as shown in Fig. 2. The voltage fed to the second grid G: is constant in phase as shown by vector E in Fi 2.

' The operation of the system disclosed here which causes demodulation of the frequency modulations to take place when one voltage ,is displaced degrees and has a superimposed phase shift with frequency variation will now be described.

i The pentode detector I 0 has a voltage fed to one grid, G1,

which has a phase given by When the frequency modulated wave given by: e1=E1 cos (wt-Fa/F: cos pt) (3) where w=21r times the carrier frequency, Fe, is applied as E in Fig. 1, the drop across tuned circuit 2 will have the phase given by (2) added to it or:

62:13: COS (Ut-Fd/Fm COS where a; and G2 are constants of the tube and er and 62 are the two grid voltages. Applying the unshifted voltage of Equation 3 to one grid,

say (hand the phase shifted voltage of (4) to the other grid, say G1, gives as the variable output:

l=sioslils cos (at-Falls cos pt) cos Simplifying and eliminating the radio frequency terms which would be eliminated by the low-pass filter in the detector output, the variable output By applying the Bessel function expansion:

sin (2: sin 40=2J1 (x) sin +2J= (x) sin a (10) the following is obtained Thuslthe output of detector It consists of the fundamental frequency, sin pt, proportional to 'a first order Bessel function of (KFo), and all the odd harmonics of the modulation frequency,

The even harmonics are absent in the output of this detector. The frequency modulations on the wave have been demodulated and appear in the output'circuit 20 of detector III. This method of demodulation has been described in Crosby application #25331 filed June 6, 1935, Patent No. 2,087,429, July 20, 1937.

If the waves fed to the two grids G1 and G1 are amplitude modulated instead of frequency modulated, the two waves fed to the grids will be e1=E1(1+m sin pt) cos wt (12) e:=Ea(1+m sin pt) cos (ot-i-I/Z) (13) where m is the amplitude modulation factor.

Substituting these in (5) gives: J;=e1o=r1s=(i+m sin m cos wt cos (-t+1r/2) (14) which when simplified gives only radio-frequency, or no detected, output. Thus under these conditions the detector is unsusceptible to amplitude modulation as long as no frequency modulation is present. With frequency modulation present, the phase angle deviates from the value 1/2 so as to let the amplitude modulation through. If the phase Shift,.r/2, is made zero,.

( 14) may be simplified to:

J=diGsE1E2(1+m sin pt)'(%+% cos 200i) (15) ascents This gives a radio-frequency term and a detected output given by:

=wb+2m sin z+ sin 2 pl] (17 The desired fundamental output is indicated bythe term "2m sin pt in (17). It will also be noted that there is a second harmonic component l2 sin 2 pt. This showsthat amplitude modulation may be received if the carrier phase shift is made zero instead of 90 degrees. This phase shift may be obtained by substituting a resistance R in place of condenser C or by the method shown in Fig. 5. It is apparent that the optimum condition for amplitude modulation reception is with a phase shift of zero (or 180, or 360, etc.) degrees and the optimum condition for frequency modulation reception is with a phase shift of 90 (or 270, etc.). Thus it can be seen that if a frequency modulated wave is being received which is also amplitude modulated, the amplitude modulation is rejected in the absence of frequency modulation, during which time the phase is 90 degrees but as frequency modulation is applied, the phase departs towards rero and 180 degrees to let amplitude modulation through as the frequency deviates from the carrier frequency.

Fig. 3 shows a push-pull arrangement using the principles used in Fig. 1. The grid G: of the second tube ll is fed from the opposite end of tuned circuit Z which is mid-tapped to feed a voltage to the first grid (3'; of detector II 180 degrees out of phase with respect to the voltage fed from Z to the first grid Ch of detector II. This reversal of phase causes the detected plate current of detector II to vary with the applied frequency in a manner opposite to that in detector iii. The detected output is obtained from resistors 20 and 20' and may be fed to a push-pull amplifier for utilization.

Fig. 4 shows a circuit operating with the same principle as Fig. 1 but utilizing a double triode type of multigrid detector ll" having a first grid G1 fed by a voltage of phase which varies as the frequency of the modulated wave supplied to tuned circuit Z varies and a second grid G: fed by a voltage from condenser 4 which is substantially the same as the voltage of the frequency modulation wave received.

I have found that this arrangement of two triodes is a linear modulator which has characteristios between its grids and plate currents which are as shown in Fig. 8. In that characteristic the grid voltage applied to the input tube is shown as the abscissae, E, and the drop across the output load impedance 2| is shown as the grid, G2, the characteristic would be as given by the curve marked +1.o.+1. The corresponding negative bias would produce the curve -1.o. It will be noted that a negative bias of 2 units causes the characteristic to have zero slope so that no output voltage appears regardless of the input voltage.

With a radio-frequency voltage applied at the input of .a pair of tubes having the character- It will be noted that the useful For instance, with zero bias on that- -istics of Fig. 8, and with that voltage large condenser ll thus-performs the same funct enough to produce saturation of the characteristics, it can be seen that the amplitude of output which occurs when saturation exists will be varied as the output grid bias is varied. This variation of bias will limit the output voltage of the tubes in a manner to cause the output to be proportional to the instantaneous value of the voltage on the output grid, Ch. I have found that thisrelationship is linear and that linear amplitude modulation may be produced which has the conventional form:

e=E(1'+Ic sin mt), sin out (18) $(wt-Fd/Fm cos pt+1r/2+KFa sin pt) (19) When these two waves are put in When (19) is simplified and radio-frequency terms eliminated, there results:

Equation 20 is of the same form as (6) which is reducible to (11). Hence, this type of detector produces the same type of output as those of Figs. 1 and 3 from which (11) is developed.

. This arrangement of two triodes is a linear modulator which has-characteristics between its grids and plate currents which are exactly similar to those obtained with the multi-grid detector of Fig. 1. The tubes used may, of course, comprise a single envelope or separate envelopes and may be triodes as shown or multiple grid tubes as used in Figs. 1 and 3.

Fig. 5 shows a utilization of the double triode' type of multi-grid detector It" with the retard branch arranged to have itsphase shifter R10 and tuned circuit Z separated by a coupling or amplifier tube 50. The wave energy is. supplied as before to leads I and 2 and passes through R1 and lead 2 to the grid 53 and cathode 55 of tube 50. The voltage is amplified in and supplied I from the anode 54 thereof to the circuit Z (parallel tuned to the mean frequency of the modulated wave) coupled by condenser 55 to the grid G1 of tube 5|. Voltage from the leads I is also supplied by lead 59 to the grid G: of tube 5| as before. The cathode 55 of tube 50 is biased by circuit 60, and the cathodes 5| of tube 5| by resistance 59.

This circuit is capable of the reception and demodulation of either frequency or amplitude modulated waves. When frequency modulation is being received, switch 8 is thrown to position F so that R1 and C form a degree phase shifter. This phase shifted voltage is amplified by tube Ill and appears across tuned circuit Z which, be-

ing parallel tuned to the mean frequency of the wave, will-impart no phase shift at the unmodulated frequency but will impart a phase shift with frequency variation as the frequency is varied. "Voltage, the phase of which is substantially unafiected, is supplied by lead 59 to the grid G2. The branch circuit including tube 50 from the input terminal of R1 to the blocking parallel resonant circuit in as elements C andZ in Fig. 1 and the phase relations are the same as portrayed by.Fig. 2. Potentiometer P allows the adjustment of the relative magnitudes of the two voltages fed to the separate grids G1 and G: of the multi-grid detector .ll.

When amplitude modulated waves are being received on the circuit of Fig. 5, switch 8 is thrown to position A. This removes the phase shift which was imparted by means of high resistance R1 and reactance C. The removal of this 90 degree phase shift allows amplitude variations of the wave applied in substantially like phase to the grids G1 and G2 to vary the detected plate current of the multi-grid detector. These amplitude variations are detected by virtue of the fact that both grids vary simultaneously instead of with a 90 degree phase relation so that amplitude variations are not balanced out. This type of detection has been described in connection with Equations 12 to 17 inclusive.

It will be apparent that the functions of the two grids of the multi-grid detectors may be interchanged. For instance, in Fig. l, the unretarded voltage may be fed to the first grid G1 and the retarded voltage fed to the second grid G2.

What is claimed is:

1. In a system for demodulatinga frequency modulated wave, asource of such waves, an electron discharge device having an output electrode, a first control electrode, a second control electrode and a cathode, means for applying frequency modulated wave energy substantially directly from said source between said second control electrode and cathode, a parallel resonant circuit and a phase shifting condenser in series across said source and adapted to be excited by said frequency modulated wave energy, a connection between spaced points onsaid parallel circuit and said first control electrode and cathode, means for biasing said first and second grid electrode negative relative to said cathode, and an output circuit connected with said output electrode and cathode.

2. Means for adapting a system as recited in claim 1 to demodulation of amplitude modulated waves comprising a resistance and means for connecting said resistance in series with said place of said condenser. I

3. In a system for demodulating a frequency modulated wave, a source of such waves, an electron discharge tube having an output electrode, a cathode and a plurality of control grid electrodes, resistive means connected between said cathode and two of said grid electrodes to bias the same negative relative to the cathode when rectification of wave energy takes place in said tube, a circuit parallel resonant to the wave energy to be demodulated connected between one of said two grid electrodes and said cathode, a nonreactive circuit connected from the source to the other of said two grid electrodes, capacitive means for applying frequency modulated wave energy to said parallel resonant circuit, said capacitive means and resonant circuit being arranged in series across said source, and a modulation frequency circuit connected to said output electrode and cathode.

4. In a system for demodulating a frequency modulated wave, a source of such waves, an electron discharge device having an output electrode,

a first control electrode. a second control elec trode and a cathode, means for applying irequency modulated wave energy directly from said source between said second control electrode and cathode, a parallel resonant circuit and a phase shifting condenser in series across said source and adapted to be excited by said frequency modulated wave energy, a connection between spaced points on said parallel resonant circuit and said first control electrode and cathode, a biasing resistance shunted by a condenser connected between said first and second control electrodes and cathode, and an output circuit connected with'said output electrode and cathode.

5. Means for adapting a system as recited in claim 3 to demodulation of amplitude modulation comprising a resistance and switching means for connecting said resistance in said series circuit in place of said phase shifting condenser.

6. In a system for demodulating a frequency modulated wave, a pair of electron discharge tube systems each having an output electrode, an electron stream source and plurality of control grid electrodes, resistive means connected be-- tween the electron stream source and two of said control grid electrodes of each of said tube systems to bias the said control grids negative relative to the cathodes when rectification of wave energy takes place in said tube systems, a reactive circuit connected in push-pull relation between corresponding control grids in each of said tube systems. a non-reactive circuit connected in push-pull relation to the control grids in each of said tube systems, means for applying frequency modulated wave energy to said reactive circuit and to said non-reactive circuit and a modulation frequency circuit connected to said output electrodes and cathode.

7. In a system for demodulating a frequency modulated wave, a plurality of electron discharge tube systems each comprising an anode, a cathode and a control grid, resistive means connected between said cathodes and each oi said control grids to bias said control grids negative relative to the cathode when rectification of wave energy takes place in said tube, a circuit parallel resonant to the wave energy to be demodulated connected between the grid and cathode of one of said systems, a non-reactive circuit connected to the grid and cathode of the-other of said tube systems, phase shifting means for impressing frequency modulated wave energy on said parallel resonant circuit, means for impressing said modulated wave on said non-reactive circuit and a modulation frequency circuit connected with the anode of one of said systems.

8. In a system for demodulating a frequency modulated wave, a plurality of electron discharge tube systems each comprising an anode, a cathode and a control grid, resistive means connected between the cathode and control grid of each of said tube systems to bias the same negative relative to the cathode when rectification of wave energy takes place in said tube systems, a source of frequency modulated wave energy, a nonreactive path coupling said source of frequency modulated wave energy to the control grid and cathode of one of said tube systems, a second path including a phase shifting circuit and a circuit parallel resonant to the frequency of the frequency modulated wave energy of said source coupling the other 0! said control grids and cathode to said source of frequency modulated wave energy and a modulation frequency circuit connected to the output electrode of one of said tube systems.

9. A system as recited in claim 8 includin means for disconnecting said phase shifting means from said second path to adapt said system to the reception of amplitude modulated waves.

MURRAY G. CROSBY. 

